The present invention relates to a method and apparatus for accurately regulating a selected operating variable of a load. The invention more particularly relates to a servo system including analog error detection as an indication of the accuracy of the regulated operating variable of the load.
A portion of a magnetic tape recording and reproducing system is described below as a preferred environment for the present invention. The invention is not limited to such recording and reproducing system environment; however, it provides a particularly representative application for the invention because of the need for very accurate regulation in order to achieve high fidelity reproduction. In most such magnetic tape recording and reproducing systems, signal transmission is achieved by rotating magnetic recording and reproducing transducer heads at high speeds in a path scanning across the magnetic tape as the path itself is longitudinally advanced past the rotating transducer assembly. The greater magnitude of head to tape speed achieved by such an arrangement has made practical the recording and reproducing of broadcast quality video signals.
Due to timing complexities and also because of the speed and phasing control requirements of the transport itself, such systems commonly include a variety of servo mechanisms and servo control systems or circuits for closely establishing stability and fidelity of the reproduced signal.
Within such recording and reproducing systems, a particularly accurate servo system is needed for the regulation of one or more operating conditions of the head and/or tape transport or the like. Such servo systems are typically employed to control both position and/or velocity operating variables of the rotating head drum motor. Position control is accomplished within the servo system by comparing a first signal, proportional to the rotation of the head, to a reference signal in order to maintain accurate control over the rotation by regulation of the motor. For example, a tach pulse may be generated by a head drum tachometer, typically one pulse for each revolution of the head drum, in order to provide a signal for comparison by the servo system.
Within a servo system employed to control position of the rotating head drum relative to the tape, the tach pulse may be compared with a reference position pulse. The reference position pulse may commonly be directly recorded upon the tape along with the recording of video signal information and thus be available during playback for access by the servo system.
With such a reference position signal or pulse being available, the interval between the occurrences of the tach pulses and a series of reference position pulses may be measured to obtain an indication of error in the position of the tape or head drum motor. It is thereafter only necessary to generate a signal representative of the position error for adjusting the motor drive and correcting the relative position of the head drum motor or the tape itself.
Other servo systems may be employed to control the operating velocity of the head drum motor. Within such servo systems, a similar error signal is generated to represent variations of the intervals between consecutive head rotation related tach pulses. The intervals between the consecutive pulses of course provide an absolute indication of the velocity for the head drum while the error signal or the change in consecutive intervals provides an indication of the rate of change in velocity.
Servo systems of the type employed for such applications may be commonly characterized as including either analog or digital components having the basic function of measuring the interval between occurrence of signals.
Analog servo systems are generally characterized by the generation of a ramp signal. The ramp signal may be employed, for example, to represent a time period which is indicated as an amplitude change, usually voltage, during the interval of the ramp. The ability to accurately measure the time period thus depends upon very accurately determining the slope of the ramp. However, the slope of the ramp is often limited because of the need for its amplitude to change linearly for the entire time interval covered by the ramp. Even further, slope variation of the ramp is common and usually result from various factors such as environmental changes including temperature increases or decreases as well as changes within the servo system itself.
Accordingly, such a simple analog based servo system may not be effective for detecting minute changes in the time period on the order of .+-. 0.02 percent, for example, as may be commonly necessary in regulating head drum velocity.
One method for adapting such an analog based servo system to provide the necessary accuracy is to increase the gain or amplitude range for the servo. In other words, the steepness of the ramp slope may be increased by providing for a greater change in voltage across the ramp. In this manner, the slope of the ramp may be made steeper in order to permit more accurate measurement. However, the voltage range inherent within such a high amplitude ramp may cause additional problems. For example, where a servo for regulating velocity is used in combination with another servo for regulating position of the head drum, the velocity servo should be AC coupled to the motor drive in order to avoid inducing instabilities within the position servo system. With such an AC coupling arrangement, the servo system is effective only to provide information as to the magnitude of velocity change. Unlike DC coupling, an AC coupling does not provide information relating to absolute velocity at any instant. This again limits the accuracy with which the motor drive may be regulated by the servo system and, in effect, partially defeats the purpose of employing a higher amplitude ramp in order to increase sensitivity of the servo system.
It has also been known to employ digital components within a servo system. For a servo system employed to control velocity of a drum head, a digital system might be employed to count reference clock pulses between successive tach pulses. A servo system employing such a digital component or binomial counter provides a precise measure of the time interval between pulses. However, such digital components tend to be characterized by a one count ambiguity. For example, a digital counter may be programmed to change count either at the rising or falling edge of a square pulse from a reference clock. When a measured signal or pulse is received at a time just prior to a change of count within the counter, the counter thereafter is reset at the subsequent count which thus closely corresponds to the actual or measured time. However, if the tach signal is received just after a change of count within the counter, the next count to which the counter is reset is almost a full count subsequent to the actual time. Accordingly, the measured count as produced by such a digital counter, at the end of a measured time interval, is only considered accurate within one count.
The count ambiguity described above has been found to be a particular problem in such servo systems since it tends to cause jitter in the regulated motor velocity or position, thus interfering with proper movement of the tape.
There has accordingly been found to remain a need for a more accurate servo system for overcoming one or more problems of the type described above.